The almost universally used process for the manufacture of alumina is the Bayer Process. In a typical commercial Bayer Process, raw bauxite is pulverized to a finely divided state. The pulverized ore is then fed to a slurry mixer where a 50% solids slurry is prepared using spent liquor and added caustic. This bauxite slurry is then diluted and sent through a series of digesters where, at about 300.degree.-800.degree. F. and 100-2000 p.s.i., 98% of the total available alumina is extracted from the ore which may contain both trihydrate and monohydrate forms of alumina. The effluent from the digesters passes through a series of flash or blow-off tanks or wherein heat and condensate are recovered as the digested slurry is cooled to about 230.degree. F. and brought to atmospheric pressure. The aluminate liquor leaving the flashing operation contains about 1-20% solids, which consist of the insoluble residues that remain after reaction between the bauxite ore and basic material used to digest the ore and the insoluble components which precipitate during digestion. The coarser solid particles are generally removed with a "sand trap" cyclone. To separate the finer solid particles from the liquor, the slurry is normally fed to the center well of a mud settler (also called a decanter, a residue thickener or a raking thickener) where it is treated with a flocculant such as a polyacrylate polymer. As the mud settles, clarified sodium aluminate solution, referred to as "green" or "pregnant" liquor, overflows a weir at the top of the mud settling tank and is passed to subsequent processing steps. The settled solids ("red mud") are withdrawn from the bottom of the mud settler and passed through a countercurrent washing circuit (called "the washer train") for further recovery of sodium aluminate and soda. Aluminate liquor overflowing the settler still contains typically 50 to 200 mg of suspended solids per liter. This liquor is then generally further clarified by filtration to give a filtrate with 10 mg. suspended solids per liter of liquor. Alumina, in relatively pure form, is then precipitated from the filtrate as alumina trihydrate crystals. The remaining liquid phase or spent liquor is returned to the initial digestion step and employed as a digestant of additional ore after being reconstituted with additional caustic. After passage through the filtration step, the level of suspended solids should be sufficiently low to provide an alumina product from the precipitation step which meets all of the industry standards.
The aforementioned insoluble components are preferably separated from the slurry at a relatively fast rate so as to make the overall Bayer Process efficient. This is generally accomplished in large settlers, decanters, residue thickeners etc, as mentioned above. The separation itself should be clean and complete with but minimal amounts of residue remaining as a dispersed phase in the solubilized aluminate liquor. The settlers, decanters, residue (or raking) thickeners (all of which am hereinafter referred to as "setter(s)" may exceed 49 m in diameter. Some are of a multideck configuration, however, single-deck units are almost exclusively used at the present time. In operation, the settler(s) feed liquor is fed at the center of the settler(s) and clarified solution overflows at the perimeter. The settler feed liquor is added to the settler(s) via a feedpipe which exits into a feedwell centrally located at the top of the settler. As the solution flows radially across the settler, the horizontal and vertical velocities become very low and the solids i.e. red mud, sink to the bottom because the specific gravity thereof is higher than the solution. The faster the settling rate, the more material which can be processed in the settler(s). The solution overflowing the settler(s) contains very few solids whereas the underflow can contain up to 35% solids. However, the overflow contains the majority of the Al.sub.2 O.sub.3 to be recovered as product. A rotating mechanism with plows mounted at an angle, slowly moves the settled solids across the bottom of the settler(s) to a discharge cone usually located in the bottom center thereof. The fine solids of the settler feed liquor settle very slowly unless accelerated by the addition of flocculants which act to bind the fine solids into flocs that often are several millimeters in diameter. The ratio of mass to drag forces is thereby increased causing the flocs to settle more rapidly. The settled solids from the settler(s) are treated in the counter-current washing circuit (washer train) discussed above, to further remove the red mud therefrom by washing. Decantation systems are employed for this purpose using washing thickeners similar in design to the settler(s). The washing operation is accomplished in as many as ten (10) stages, the solids moving counter-current to the wash stream to recover solubles and the overflow being recycled back from each stage of the washer train to its predecessor. Flocculant addition to the stages of the washer train increases the solids separation, it being generally known to utilize different flocculants in the earlier stages from those used in the latter stages, see, for example, U.S. Pat. No. 4,678,585, hereby incorporated herein by reference. The settler(s) overflow must be clarified in order to recover as pure a product as possible. The overflow is subjected to what is called polish filtration (sometimes referred to as clear pressing) whereby the solids remaining are removed. Stationary filters are also used for this purpose. It is normal to attempt to reduce the solids to below about 0.5 mg/L to enable product purity to be as high as possible since the ultimate product is used for applications where purity is essential, e.g. toothpaste.
The reduction of the amount of suspended solids in the stream comprising the Bayer Process settler(s) overflow and that which proceeds to the initial stages of the washer train, ie. the first separation stages to which the red mud slurry from the last settler are introduced, has been a major problem for many years.
Among the methods of overcoming the above problems and materially speeding up separation of suspended solids from Bayer Process streams as well as effecting a cleaner separation of the constituents, is that disclosed in U.S. Pat. No. 4,767,540, issued on Aug. 30, 1988. In this patent there is disclosed the addition of a water-soluble polymer containing pendant hydroxamic acid or salt groups to Bayer Process streams, alone or in conjunction with an anionic flocculant. The polymer containing hydroxamic acid groups may be added into the Bayer Process caustic aluminate process stream alone, subsequent to, followed by or in association with the anionic flocculent, e.g. a conventional polyacrylate polymer. This treatment is shown to reduce levels of suspended solids in the process stream as compared to then existing state-of-the-art processes.
Said U.S. Pat. No. 4,767,540, however, does not disclose the use of a lower molecular weight hydroxamated polymer followed by a blend of an anionic flocculant and a higher molecular weight hydroxamated polymer in accordance with the present invention.